1) Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a metal casing and a metal-made honeycomb core body for carrying an exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor vehicle, the exhaust gas cleaning device is generally installed at an intermediate point of an exhaust pipe.
More specifically, this invention is concerned with an exhaust gas cleaning device of the above sort, which is employed under severe conditions. The exhaust gas cleaning device features the use of a cylindrical metal casing which defines a cavity compartment isolated from an exterior so as to improve the efficiency of cleaning of exhaust gas by the honeycomb core body.
2) Description of the Related Art
Exhaust gas cleaning devices of the above sort have conventionally been fabricated in the following manner. Namely, a planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy or sinuous form are superposed one over the other in a contiguous relation and rolled together into a multilayered spiral form or are superposed in layers, thereby forming a honeycomb-shaped multilayered composite body (hereinafter called the "honeycomb core body") axially defining a number of network-patterned gas flow passages for allowing exhaust gas to flow therethrough.
The honeycomb core body fabricated as described above is then inserted into a cylindrical metal casing which are opening in both ends thereof. Incidentally, as the material of the metal casing, a heat-resistant stainless steel sheet whose thickness is 1-2 mm is generally used.
The honeycomb core body and metal casing are thereafter firmly joined together by brazing or welding, so that they can withstand thermal deformation forces due to the high temperature of exhaust gas, exothermic reactions of exhaust gas induced by the catalyst and further vibrations while the associated motor vehicle is running. Needless to say, the planar and corrugated bands which make up the honeycomb core body can be firmly joined together at contacts therebetween by any one of various suitable methods, for example, by brazing or welding.
In an exhaust gas cleaning device of the above sort which is constructed of a metal-made honeycomb core body and a cylindrical metal casing, an outer peripheral wall of the metal casing is in contact with the surrounding air so that the internal heat, namely, the heat of the honeycomb core body is radiated from the metal casing. A substantial temperature difference is therefore developed between the outer peripheral wall of the metal casing and the honeycomb core body inside the metal casing. As a result, thermal deformation forces from the honeycomb core body are concentrated around areas of contact between the inner peripheral wall of the metal casing and an outer peripheral wall of the honeycomb core body. In directions perpendicular to the longitudinal axis of the honeycomb core body (hereinafter called "radial directions of the honeycomb core body"), to say nothing of the axial direction of the honeycomb core body (namely, the direction in which exhaust gas flows in and passes through), deformation forces are produced by thermal expansion and thermal stress occurred in an atmosphere of high temperature due to the elevated temperature of exhaust gas itself and heat generated by a catalytic reaction of unburnt gas. These deformation forces then concentrate in the vicinity of an outer peripheral portion of the honeycomb core body, said outer peripheral portion having a large temperature gradient, or around the areas of contact between the outer peripheral wall of the honeycomb core body and the inner peripheral wall of the associated metal casing by way of the elements (i.e., the planar band and corrugated band) of the honeycomb core body.
In the vicinity of the outer peripheral portion of the honeycomb core body or around the areas of contact between the outer peripheral wall of the honeycomb core body and the inner peripheral wall of the metal casing, the large deformation forces caused by the aforementioned thermal expansion and thermal stress cause cracking and breakage of the planar band and/or corrugated band, which make up the honeycomb core body, and also dropping or separation at the areas of contact between the respective bands and also at the areas of contact between the honeycomb core body and the metal casing.
In addition, cracking and breakage also occur in the outer peripheral wall of the metal casing itself, which is generally 1-2 mm thick as described above. This is not only very dangerous but also leads to a reduction in the service life of the exhaust gas cleaning device.
Corrections are therefore needed to the conventional belief that it is only necessary to establish firm joining or unification between a planar band and an associate corrugated band, which make up the honeycomb body, or between the outer peripheral wall of the honeycomb body and the inner peripheral wall of an associated metal casing.